The international date line is an imaginary line that for the most part is at ±180° Longitude, but has an odd shape to pass around Russia and islands in the Pacific. It is on the side of the Earth that lies opposite the prime meridian. Its purpose is to offset the hours that are added as one travels east through each successive time zone.
The first phenomenon to occur in association with the date-line problem was part of Magellan's circumnavigation of the globe. The crew returned to a Spanish stopover sure of the day of the week, as attested by various carefully maintained sailing logs. Nevertheless, those on land insisted the day was different. Although readily understandable, this phenomenon caused great excitement at the time, to the extent that a special delegation was sent to the Pope to explain this oddity to him.
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The effect of ignoring the date line is also seen in Jules Verne's work of fiction Around the World in Eighty Days, in which the travellers return to London after a trip around the world, thinking that they have lost the bet that is the central premise of the story. Having circumnavigated in the direction opposite Magellan's, they believe the date there to be one day later than what it truly is.
Anyone travelling west and passing the line must add a day to what they would otherwise expect the date and time to be. Correspondingly, those going east must subtract a day. Magellan's crew and Verne's travellers neglected those adjustments, respectively.